render-diff composes without the participants list deploy-kit requires #198

What's wrong

actions/render-diff/run.sh calls deploy-kit compose without --participants. The composer has required that option since deploy-kit 0.10.0 (JorisJonkers-dev/deploy-kit#250), and 1.0.0 still does:

deploy-kit compose --fragments <directory> --participants <file> --cluster-state <file> ...

So both sides fail at argument parsing. A pull request's render diff then reports "This change does not compose" for reasons unrelated to the change. The tests don't catch it because they run against a stub command.

What to build

• Pass --participants "$estate/participants.yml", the Estate repository's list, which it already checks out.
• Make the stub in tests/test_render_diff.py refuse a compose call that omits --participants, the same way the real CLI does, so the test fails until the fix is in.

Acceptance criteria

• On a real pull request, render-diff composes both sides with the pinned toolkit and posts a diff.
• A test fails when --participants is missing.

Found while pinning deploy-kit 1.0.0 (#197). Part of JorisJonkers-dev/deploy-kit#195.